As a method for forming a film on a semiconductor wafer (hereinafter, referred to as “wafer”) as a substrate, there is known a method referred to as a so-called ALD (atomic layer deposition) method or the like in which a thin film is formed by sequentially supplying a source gas and a reactant gas that reacts with the source gas to the wafer and depositing a molecular layer of by-product on a surface of the wafer.
In the ALD method, it is required to supply a replacement gas for replacing an atmosphere between the supply of the source gas and the supply of the reactant gas. In order to obtain a high throughput, the replacement of the atmosphere needs to be quickly carried out and a formed film needs to have good in-plane uniformity.
Recently, a film of nanometer order may require in-plane thickness uniformity in the wafer surface, e.g., about 2% or less based on 1σ %, to be described later, which is percentage obtained by dividing standard deviation σ by average. Accordingly, it is required to develop a gas supply structure capable of providing good replacing performance and realizing film formation while ensuring good in-plane uniformity.
As the gas supply structure, there is known an apparatus for supplying a processing gas into a processing chamber through a shower head, which is disclosed in Japanese Patent Application Publication No. 2013-165276. However, in this apparatus, there is problem that dispersiveness of the processing gas in the shower head is poor and hence the in-plane thickness uniformity of the formed film is poor.
In addition, there is known a film forming apparatus, disclosed in Japanese Patent Application Publication No. 2014-070249, which includes a gas dispersion unit that horizontally discharges a gas into a diffusion space by using a shower head having good replacement efficiency due to a small diffusion space. In this apparatus, flow direction of the discharged gas is changed and then, the discharged gas passes through a bottom surface of the shower head. However, in this apparatus, there are problems that the gas flow is poor in a region below the gas dispersion unit provided at the center of the diffusion space, so that film thickness difference increases between a central portion and a peripheral portion, which makes it difficult to improve the in-plane uniformity of the film thickness, and that the supply of the processing gas is not sufficient at the peripheral portion of a substrate, so that a film thickness at the peripheral portion of the substrate is not stable.